Lithium secondary batteries are now widely used in consumer electronics. They benefit from the light weight of Li and from its strong reducing character, thus providing the highest power and energy density among known rechargeable battery systems. Lithium secondary batteries are of various configurations depending on the nature of the electrode materials and of the electrolyte used. The commercialised Li-ion system, for instance, uses LiCoO2 and Carbon graphite as positive and negative electrodes, respectively with LiPF6 in EC/DEC/PC as a liquid electrolyte. The operating voltage of the battery is related to the difference between thermodynamic free energies within the negative and positive electrodes. Solid oxidants are therefore required at the positive electrode, the materials of choice, up to now, being either the layered LiMO2 oxides (with M is Co or Ni) or the 3-dimensional spinel structure of Li[Mn2]O4. Extraction of Li from each of these three oxides gives access to M4+/M3+ redox couples located between 3.5 to 5 V vs. Li+/Li.
Three-dimensional framework structures using (XO4)n− polyanions have been proposed recently (U.S. Pat. No. 5,910,382) as viable alternatives to the LiMxOy oxides. LiFePO4 and Li3Fe2(PO4)3 in particular are the most promising Fe-containing materials that can work at attractive potentials vs. Li+/Li (3.5 V and 2.8 V respectively). Both compounds operate on the Fe3+/Fe2+ redox couple which take advantage from the inductive effect of the XO4n− groups that diminishes the strength of the Fe—O bond compared to a simple oxide.
Pioneering work by Padhi (Padhi et al., J. Elec. Soc. 144(4)) demonstrated the reversible extraction of Li from the olivine-structured LiFePO4 prepared by solid state reaction at 800° C. under Ar atmosphere, starting from Li2CO3 or LiOH.H2O, Fe(CH3COO)2 and NH4H2PO4.H2O. Unfortunately, probably due to kinetic limitations of the displacement of the LiFePO4/FePO4 interface, only 60-70% of the theoretical capacity of 170 mAh/g of active material, was achieved, whatever the charge or discharge rate applied. Indeed, the use of high synthesis temperatures leads to the formation of large particles in which ionic and electronic conductivity is the limiting factor. Several research groups recently reported improvements in the effective reversible capacity of LiFePO4 by decreasing the particle size. This can be done by using highly reactive FeII precursors (JP 2000-294238 A2), or by using a solution route (WO 02/27824 AI), thus allowing LiFePO4 formation at lower temperatures compared to the solid state route described by Padhi.
The poor electronic conductivity of the product can be improved by coating the particles with conductive carbon. This has been done by ball milling LiFePO4 and carbon (Huang et al., Electrochem. Solid-State Lett., 4, A170 (2001)) or by adding a carbon containing compound to already made LiFePO4 and proceeding to a subsequent calcination at about 700° C. (CA 2,270,771). Carbon, and preferably amorphous carbon, can also be introduced in the LiFePO4 synthesis process, being mixed with the solid synthesis precursors before calcination (EP 1184920 A2).
The main problems that may jeopardise the effective use in a positive electrode for Li batteries of Li-containing olivine or NASICON powders such as LiFePO4 or other components mentioned by Goodenough et al. in U.S. Pat. No. 5,910,382, arises from their low electronic conductivity and from the fact that both end-members of the de-intercalation process (e.g. LiFePO4 and FePO4) are poor ionic conductors.
As described above, adding carbon, thereby coating the particles with a conductive layer, alleviates the electronic conductivity problem. However, high amounts of carbon are needed. Whereas carbon does not participate in the redox reactions useful for the operation of the battery, a strong penalty for the overall specific capacity of the composite positive electrode is paid. This is illustrated in JP 2000-294238 A2 wherein a LiFePO4/Acetylene Black ratio of 70/25 is used.
The ionic conduction problem can be solved by producing very fine-grained particles. Using a solution route synthesis has been found to be advantageous compared to the classic solid synthesis route. This solution route has been described in EP1261050. This route provides for a very finely divided, homogeneous precursor which needs only moderate conditions of temperature and time to react to the desired crystalline structures. Thanks to the moderate conditions, grain growth, leading to unwanted coarse particles, is avoided. After synthesis, such a powder has to be ball-milled with a relatively large quantity of conductive carbon, typically amounting to 17 wt. %.